Behavior of Li abundances in solar-analog stars^*

II. Evidence of the connection with rotation and stellar activity

¹ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan e-mail: [takeda.yoichi;kawanomoto.satoshi;ando.hys]@nao.ac.jp,e-mail: sakurai@solar.mtk.nao.ac.jp
² Gunma Astronomical Observatory, 6860-86 Nakayama, Takayama-Mura, Agatsuma-gun, Gunma 377-0702, Japan e-mail: honda@astron.pref.gunma.jp

Received: 18 December 2009
Accepted: 2 February 2010

Abstract

Context. We previously attempted to ascertain why the Li i 6708 line-strengths of Sun-like stars differ so significantly despite the superficial similarities of stellar parameters. We carried out a comprehensive analysis of 118 solar analogs and reported that a close connection exists between the Li abundance (A_Li) and the line-broadening width (v_r+m; mainly contributed by rotational effect), which led us to conclude that stellar rotation may be the primary control of the surface Li content.

Aims. To examine our claim in more detail, we study whether the degree of stellar activity exhibits a similar correlation with the Li abundance, which is expected because of the widely believed close connection between rotation and activity.

Methods. We measured the residual flux at the line center of the strong Ca ii 8542 line, r₀(8542), known to be a useful index of stellar activity, for all sample stars using newly acquired spectra in this near-IR region. The projected rotational velocity (v_e sin i) was estimated by subtracting the macroturbulence contribution from v_r+m that we had already established.

Results. A remarkable (positive) correlation was found in the A_Li versus (vs.) diagram as well as in both the r₀(8542) vs. v_e sin i and A_Li vs. v_e sin i diagrams, as had been expected. With the confirmation of rotation-dependent stellar activity, this clearly shows that the surface Li abundances of these solar analogs progressively decrease as the rotation rate decreases.

Conclusions. Given this observational evidence, we conclude that the depletion of surface Li in solar-type stars, probably caused by effective envelope mixing, operates more efficiently as stellar rotation decelerates. It may be promising to attribute the low-Li tendency of planet-host G dwarfs to their different nature in the stellar angular momentum.